1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly to an active matrix type in-plane switching liquid crystal display device.
2. Description of the Related Art
Recently, there has been developed an in-plane switching type liquid crystal display device in which molecular axes of aligned liquid crystal molecules are rotated in a plane parallel to a substrate, to thereby display images.
In an in-plane switching type liquid crystal display device, since a viewer looks only at minor axes of liquid crystal molecules even if he/she turns his/her viewpoint, an angle of visibility is not dependent on an inclination of liquid crystal molecules. Hence, an in-plane switching type liquid crystal display device can present a wider angle of visibility than a conventional liquid crystal display device such as a twisted nematic (TN) mode liquid crystal display device where an electric field is generated between substrates sandwiching a liquid crystal layer therebetween in a direction perpendicular to the substrates.
In an in-plane switching type liquid crystal display device, a plurality of scanning lines and signal lines are arranged on one of transparent substrates sandwiching a liquid crystal layer therebetween. Thin film transistors (TFTs) are arranged at intersections of the scanning and signal lines. Sources of the thin film transistors are electrically connected to pixel electrodes. Opposing electrodes are positioned in facing relation with the pixel electrodes.
When an image is displayed on a display screen in in-plane switching type liquid crystal display device, a voltage is applied to the scanning lines for successively turning the thin film transistors on, and then, a voltage having a magnitude determined in accordance with a gradation to be displayed is applied to an associated pixel electrode through the thin film transistor from a data line. As a result, there is produced an electric field between the pixel and opposing electrodes in parallel with the transparent substrates. The thus produced electric field varies a direction of alignment of liquid crystal molecules in the liquid crystal layer, and resultingly, vary optical characteristics of liquid crystal, ensuring a desired gradation.
The above-mentioned in-plane switching type liquid crystal display device is accompanied with a problem that flicker occurs when a certain image is displayed for a certain period of time, and thereafter, the image is switched into another image in which all pixels are arranged to be in the same gradation.
For instance, it is assumed that a liquid crystal display device is driven in accordance with a dot inversion driving method in which a voltage for driving a positive polarity and a voltage for driving a negative polarity are switched to each other in each of pixels at a predetermined interval. In this case, the above-mentioned problem occurs in particular when a checker pattern in which black-displaying pixels B (minimum gradation) and white-displaying pixels W (maximum gradation) are alternately arranged in a matrix, as illustrated in FIG. 7, is displayed in a certain period of time, and thereafter, all pixels are switched into images having the same gradation.
For another instance, it is assumed that a liquid crystal display device is driven in accordance with a line inversion driving method in which a voltage for driving a positive polarity and a voltage for driving a negative polarity are switched to each other in every lines at a predetermined interval. In this case, the above-mentioned problem occurs in particular when a stripe pattern in which black-displaying and white-displaying pixels are alternately arranged in every two lines is displayed in a certain period of time, and thereafter, all pixels are switched into images having the same gradation.
In view of the above-mentioned problems in the conventional liquid crystal display devices, it is an object of the present invention to provide an in-plane switching type liquid crystal display device which is capable of reducing flickers in a display screen It is also an object of the present invention to provide a method of driving a liquid crystal display device which method is capable of reducing flickers in a display screen.
In one aspect of the present invention, a liquid crystal display device includes (a) a first substrate, (b) a second substrate, (c) a liquid crystal layer sandwiched between the first and second substrates, (d) a plurality of scanning lines arranged on the first substrate, (e) a plurality of signal lines arranged on the first substrate, (f) a plurality of first switches arranged at intersections of the scanning lines and the signal lines, (g) a plurality of pixel electrodes each electrically connected to each of the first switches, (h) a plurality of opposing electrodes each arranged in parallel with each of the pixel electrodes, and (i) a signal line driver which switches a first voltage for driving a positive pole and a second voltage for driving a negative pole at a predetermined interval in accordance with a gradation, and outputs the positive or negative driving voltage to the signal lines, the signal line driver compensating for the first and second voltages such that averages of the first and second voltages in each of gradations are different from one another.
For instance, the signal line driver may be designed to compensate for the first and second voltages such that an average of the first and second voltages is smaller in a higher gradation.
For instance, the signal line driver may be designed to compensate for the first and second voltages such that a difference between an average of positive and negative voltages to be applied to the pixel electrode in association with a gradation and a voltage of the opposing electrode associated with the pixel electrode is kept substantially constant irrespective of the gradation.
For instance, such a voltage may be applied to the opposing electrodes that a flicker is not allowed to occur in a display where pixels displaying intermediate gradation and pixels displaying black are alternately arranged. For instance, the signal line driver may be designed to compensate for the first and second voltages such that a difference between an average of the first and second voltages, associated with a maximum gradation, and an average of the first and second voltages, associated with a minimum gradation, is in the range of xe2x88x921.0 to 0.0 volts both inclusive.
For instance, the signal line driver may be designed to compensate for the first and second voltages such that a difference between an average of the first and second voltages, associated with a maximum gradation, and an average of the first and second voltages, associated with a minimum gradation, is in the range xe2x88x920.9 to xe2x88x920.2 volts both inclusive.
For instance, the signal line driver may be designed to compensate for the first and second voltages such that a difference between an average of the first and second voltages, associated with a maximum gradation, and an average of the first and second voltages, associated with a minimum gradation, is in the range of xe2x88x920.5 to xe2x88x920.3 volts both inclusive.
It is preferable that the liquid crystal display device further includes a light barrier which does not allow a light to reach the first switches.
It is preferable that liquid crystal in the liquid crystal layer has a specific resistance in the range of 4.5xc3x971010 xcexa9 cm and 2.0xc3x971013 xcexa9 cm both inclusive, preferably in the range of 3.0xc3x971011 xcexa9 cm and 1.0xc3x971013 xcexa9 cm both inclusive, and more preferably in the range of 5.0xc3x971011 xcexa9 cm and 2.0xc3x971012 xcexa9cm both inclusive.
There is further provided a liquid crystal display device includes (a) a first substrate, (b) a second substrate, (c) a liquid crystal layer sandwiched between the first and second substrates, (d) a plurality of scanning lines arranged on the first substrate, (e) a plurality of signal lines arranged on the first substrate, a plurality of first switches arranged at intersections of the scanning lines and the signal lines, (g) a plurality of pixel electrodes each electrically connected to each of the first switches, (h) a plurality of opposing electrodes each arranged in parallel with each of the pixel electrodes, (i) a signal line driver which switches a first voltage for driving a positive pole and a second voltage for driving a negative pole at a predetermined interval in accordance with a gradation, and outputs the positive or negative driving voltage to the signal lines, and (j) a reference driving voltage supplier which generates first and second reference driving voltages both compensated for in each of gradations, and associated with at least one specific gradation, the signal line driver compensating for the first and second voltages such that averages of the first and second voltages in each of gradations are different from one another, the signal line driver including a driving voltage calculator which receives at least one pair of the first and second reference driving voltages from the reference driving voltage supplier, and calculates and outputs the first and second reference driving voltages associated with a gradation to be displayed, based on the received first and second reference driving voltages.
There is still further provided a liquid crystal display device includes (a) a first substrate, (b) a second substrate, (c) a liquid crystal layer sandwiched between the first and second substrates, (d) a plurality of scanning lines arranged on the first substrate, (e) a plurality of signal lines arranged on the first substrate, (If a plurality of first switches arranged at intersections of the scanning lines and the signal lines, (g) a plurality of pixel electrodes each electrically connected to each of the first switches, (h) a plurality of opposing electrodes each arranged in parallel with each of the pixel electrodes, (i) a signal line driver which switches a first voltage for driving a positive pole and a second voltage for driving a negative pole at a predetermined interval in accordance with a gradation, and outputs the positive or negative driving voltage to the signal lines, and (j) a reference driving voltage supplier which generates first and second reference driving voltages both compensated for in each of gradations, the signal line driver compensating for the first and second voltages such that averages of the first and second voltages in each of gradations are different from one another, the signal line driver including a driving voltage selector which receives the first and second reference driving voltages from the reference driving voltage supplier, and selects and outputs first and second reference driving voltages associated with a gradation to be displayed, among the received first and second reference driving voltages.
There is yet further provided a liquid crystal display device includes (a) a first substrate, (b) a second substrate, (c) a liquid crystal layer sandwiched between the first and second substrates, (d) a plurality of scanning lines arranged on the first substrate, (e) a plurality of signal lines arranged on the first substrate (f) a plurality of first switches arranged at intersections of the scanning lines and the signal lines, (g) a plurality of pixel electrodes each electrically connected to each of the first switches, (h) a plurality of opposing electrodes each arranged in parallel with each of the pixel electrodes, and (i) a signal line driver which switches a first voltage for driving a positive pole and a second voltage for driving a negative pole at a predetermined interval in accordance with a gradation, and outputs the positive or negative driving voltage to the signal lines, the signal line driver compensating for the first and second voltages such that averages of the first and second voltages in each of gradations are different from one another, the signal line driver including (i-1) a memory storing first and second voltages having been compensated for in each of gradations, (i-2) a driving voltage detector which receives the first and second voltages in a digital form from the memory, in association with a gradation to be displayed, and outputs the thus received digital first and second voltages, and (i-3) a digital-analog converter which receives the digital first and second voltages from the driving voltage detector, converts the thus received digital first and second voltages into analog first and second voltages, and outputs the analog first and second voltages.
There is still yet further provided a liquid crystal display device includes (a) a first substrate, (b) a second substrate, (c) a liquid crystal layer sandwiched between the first and second substrates, (d) a plurality of scanning lines arranged on the first substrate, (e) a plurality of signal lines arranged on the first substrate, (f) a plurality of first switches arranged at intersections of the scanning lines and the signal lines, (g) a plurality of pixel electrodes each electrically connected to each of the first switches, (h) a plurality of opposing electrodes each arranged in parallel with each of the pixel electrodes, and (i) a signal line driver which switches a first voltage for driving a positive pole and a second voltage for driving a negative pole at a predetermined interval in accordance with a gradation, and outputs the positive or negative driving voltage to the signal lines, the signal line driver compensating for the first and second voltages such that averages of the first and second voltages in each of gradations are different from one another, the signal line driver including (i-1) a driving voltage supplier which generates a driving voltage in accordance with a gradation to be displayed, (i-2) a compensation supplier which generates a compensation voltage associated with the gradation, and (i-3) an adder which adds the driving voltage transmitted from the driving voltage supplier and the compensation voltage transmitted from the compensation supplier to each other.
There is further provided a liquid crystal display device includes (a) a first substrate, (b) a second substrate, (c) a liquid crystal layer sandwiched between the first and second substrates, (d) a plurality of scanning lines arranged on the first substrate, (e) a plurality of signal lines arranged on the first substrate (f) a plurality of first switches arranged at intersections of the scanning lines and the signal lines, (g) a plurality of pixel electrodes each electrically connected to each of the first switches, (h) a plurality of opposing electrodes each arranged in parallel with each of the pixel electrodes, (i) a signal line driver which switches a first voltage for driving a positive pole and a second voltage for driving a negative pole at a predetermined interval in accordance with a gradation, and outputs the positive or negative driving voltage to the signal lines, and (j) a light source positioned at the opposite side of the liquid crystal layer about the first substrate, and (k) a brightness detector which detects a brightness of light emitted from the light source, the signal line driver compensating for the first and second voltages such that averages of the first and second voltages in each of gradations are different from one another, the signal line driver including a compensator which further compensates for the compensated first and second voltages, based on the brightness detected by the brightness detector.
It is preferable that the signal line driver compensates for the first and second voltages by adding a compensation voltage to the first and second voltages, the compensation voltage V1 being defined in accordance with the following equation:
Vi=Vxc3x97(xe2x88x926.66xc3x9710xe2x88x925xc3x97(Xxe2x88x920.47)) 
wherein V indicates a compensation voltage to be obtained when the brightness is maximum, and X indicates the brightness detected by the brightness detector.
For instance, the brightness detector may be designed to detect a current to be supplied to the light source, in place of the brightness.
It is preferable that the compensator compensates for the compensated first and second voltages by adding a compensation voltage to the compensated first and second voltages, the compensation voltage V1 being defined in accordance with the following equation:
Vi=Vxc3x97(0.22xc3x97(X+2.0)) 
wherein V indicates a compensation voltage to be obtained when the brightness is maximum, and X indicates the current detected by the brightness detector.
There is further provided a liquid crystal display device includes (a) a first substrate, (b) a second substrate, (c) a liquid crystal layer sandwiched between the first and second substrates, (d) a plurality of scanning lines arranged on the first substrate, (e) a plurality of signal lines arranged on the first substrate, (f) a plurality of opposing electrode lines arranged on the first substrate, (g) a plurality of first switches arranged at intersections of the scanning lines and the signal lines, (h) a plurality of second switches each positioned in the vicinity of each of the first switches, (i) a plurality of pixel electrodes each electrically connected to each of the first switches, and (k) a plurality of opposing electrodes each electrically connected to each of the second switches and each arranged substantially in parallel with each of the pixel electrodes.
In another aspect of the present invention, there is provided a method of driving a liquid crystal display device includes (a) a first substrate, (b) a second substrate, (c) a liquid crystal layer sandwiched between the first and second substrates, (d) a plurality of scanning lines arranged on the first substrate, (e) a plurality of signal lines arranged on the first substrate, (f) a plurality of first switches arranged at intersections of the scanning lines and the signal lines, (g) a plurality of pixel electrodes each electrically connected to each of the first switches, and (h) a plurality of opposing electrodes each arranged in parallel with each of the pixel electrodes, the method includes the steps of (a) compensating for first and second voltages such that averages of the first and second voltages in each of gradations are different from one another, and (b) outputting the thus compensated first and second voltages to the signal lines.
It is preferable that the first and second voltages are compensated for in the step (a) such that an average of the first and second voltages is smaller in a higher gradation.
It is preferable that the first and second voltages are compensated for in the step (a) such that a difference between an average of positive and negative voltages to be applied to the pixel electrode in association with a gradation and a voltage of the opposing electrode associated with the pixel electrode is kept. substantially constant irrespective of the gradation.
The method may further include the step of applying such a voltage to the opposing electrodes that a flicker is not allowed to occur in a display where pixels displaying intermediate gradation and pixels displaying black are alternately arranged.
It is preferable that the first and second voltages are compensated for in the step (a) such that a difference between an average of the first and second voltages, associated with a maximum gradation, and an average of the first and second voltages, associated with a minimum gradation, is in the range of xe2x88x921.0 to 0.0 volts both inclusive, preferably in the range of xe2x88x920.9 to xe2x88x920.2 volts both inclusive, and more preferably in the range of xe2x88x920.5 to xe2x88x920.3 volts both inclusive The method may further include the step of generating first and second reference driving voltages both compensated for in each of gradations, and associated with at least one specific gradation, and wherein the step (a) includes the steps of receiving at least one pair of the first and second reference driving voltages from the reference driving voltage supplier, and calculating and outputs the first and second reference driving voltages associated with a gradation to be displayed, based on the received first and second reference driving voltages.
The method may further include the step of generating first and second reference driving voltages both compensated for in each of gradations, and wherein the step (a) includes the steps of receiving the first and second reference driving voltages from the reference driving voltage supplier, and selecting and outputs first and second reference driving voltages associated with a gradation to be displayed, among the received first and second reference driving voltages.
For instance, the step (a) may be designed to include the steps of storing first and second voltages having been compensated for in each of gradations, receiving the first and second voltages in a digital form, in association with a gradation to be displayed, outputting the thus received digital first and second voltages, receiving the digital first and second voltages, converting the thus received digital first and second voltages into analog first and second voltages, and outputting the analog first and second voltages.
For instance, the step (a) may be designed to include the steps of generating a driving voltage in accordance with a gradation to be displayed, generating a compensation voltage associated with the gradation, and adding the driving voltage and the compensation voltage to each other.
The method may further include the steps of detecting a brightness of a light reaching the liquid crystal layer, and compensating for the first and second voltages both having been once compensated for, based on the brightness detected by the brightness detector.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the present invention, first and second voltages to be applied to signal lines are compensated for such that averages of the first and second voltages in each of gradations are different from one another. This makes it possible to avoid variance in a liquid crystal capacity, caused by variance in a gradation, and variance in field-through caused by a leakage current in thin film transistors. As a result, the present invention can prevent flickers in a display screen even if any images are displayed, and ensure high quality in displayed images.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.